Parts and components made of high-strength alloys for use in aircraft, space vehicles and power generation equipment generally are more susceptible to failure due to cracking than parts and components made from lower strength alloys. Those cracks typically propagate from defects such as nonmetallic inclusions resulting primarily from the alloy making processes and cracks resulting from component fabrication processes such as welding and grinding.
All steel and superalloy ingots contain to some degree nonmetallic matter, i.e., inclusions, consisting almost exclusively of oxides, with lesser amounts of sulphides, in various combinations and mixtures with each other. Such inclusions are derived chiefly from the oxidizing reactions of the refining process and the deoxidizing materials added to the alloy in the furnace, ladle, or molds. A few inclusions may also result from erosion of the refractories used to line the vessels used in alloy making processes.
Given that all metallic components can contain inclusions and cracks, there is a design trade-off between alloy strength and component weight which is influenced to no small degree by the ability to reliably and nondestructively detect defects as small as those which can cause failure.
For example, the life of gas turbine disks made from high-strength superalloys can be limited by low-cycle fatigue (LCF) endurance. First failures in LCF are typically initiated at surface, or near-surface, inclusions and features, e.g., carbides, as small as 5-100 square mils in size. Presently, the only method used for quality control is low-cycle fatigue tests on samples machined from disks. The LCF testing technique suffers from two major drawbacks, both related to its destructive nature: (1) it is expensive and (2) it cannot be applied to every single part produced. A more desirable approach would be a fast, nondestructive surface inclusion-related technique which could be used on the finished component or on sections of the billet from which the components are to be produced.